This invention relates to nanopositioning such as for an atomic force microscope, and more particularly to such a nanopositioning system using series-connected actuators that simultaneously achieve high speed performance as well as large out-of-plane and lateral scan ranges.
High speed imaging capability expands the applications of atomic force microscopy (AFM) to the study of dynamic nano-scale processes. This advantage and the associated research potentials [1, 2, 3] have been the motivation behind a considerable amount of research efforts on high-speed atomic force microscopy during the past decade [4, 5, 6, 7, 8, 9, 10]. These efforts have brought about significant improvements in the state of the art and have unlocked novel scientific observations [11]. To enable high-speed AFM imaging, electrical [12], optical [13], mechanical [14, 15] and control [16, 17, 18, 19] components of the AFM have been improved.
Research on the design of AFM scanners [20] has led to rigid designs capable of high scan speeds. Optimal feedforward and feedback control techniques are used to reduce the tip-sample interaction forces at high speeds [21]. Active vibration suppression techniques have been applied to tackle the out-of-plane scanner dynamics and extend the closed loop bandwidth of the AFM [16]. The size of AFM micro-cantilevers has been reduced significantly, increasing the probe resonance frequency to a few megahertz while maintaining small spring constant for minimal tip-sample interaction forces [22]. Furthermore, to enable application of these small probes, the optical beam deflection setup has been modified to achieve a smaller focused laser 15 spot size [13, 23].
Increasing the bandwidth performance of the AFM scanner while maintaining a reasonable scan range is currently the most challenging aspect of high-speed atomic force microscopy. This limitation is rather fundamental. Wider mechanical bandwidth requires increased rigidity and reduced translated mass, and leads naturally to decreased lateral and out-of-plane scan range [15, 22]. This has limiting practical implications. Small out-of-plane range limits the application of AFM as the topography height variations due to sample tilt [17] or thickness e.g. cells [24] may necessitate several microns of travel. Limited lateral range is likewise problematic as sample features of interest can span a large area in many imaging applications. To simultaneously achieve both the range and speed requirements of the out-of-plane AFM actuator, researchers have applied dual actuation methodologies [17, 25, 26, 27, 28, 29, 30, 31, 32, 33]. In this approach, emanating from hard disk drive (HDD) research [34], two out-of-plane nano-positioners are combined where one is fast and short-range and the other slow and large-range. These earlier works on multi-actuation, all limited to out-of-plane motion of the scanner, can be divided into two main categories. In one, self-actuated AFM probes are used in combination with external piezos [28,29]. This approach suffers from either the bandwidth or range limitations of bimorph actuators [28] or the complexity of attachment and actuation of magnetic nanoparticles [29]. In the second category [25, 26, 27, 30], two external piezo actuators are used on independent substrates where one moves the sample and the other moves the probe. This arrangement avoids dynamic coupling, but limits the technique to only two actuators and requires modifications in the optical path of sample-scan AFMs.
An object of the present invention is a new multi-actuated atomic force microscope which features a number of practical advantages. The concept of multi-actuation is extended to all scan directions, enabling large-range and high speed performance for both lateral and out-of-plane actuators. The method is presented in a generalized form applicable to any number of actuators.